Extreme ultraviolet/soft X-ray laser nano-scale patterning using the demagnified Talbot effect

ABSTRACT

An apparatus and method for nanopatterning of substrates using the demagnified Talbot effect, wherein: (a) large arrays of nanostructures can rapidly be printed; (b) short extreme ultraviolet wavelengths permits sub-100 nm spatial resolution; (c) the de-magnification factor can be continuously adjusted, that is, continuously scaled; (d) the patterning is the effect of the collective diffraction of numerous tiled units that constitute the periodic array, giving rise to error resistance such that a defect in one unit is averaged over the area of the mask and the print does not show any defects; (e) the Talbot mask does not wear out since the method is non-contact; and (f) the feature sizes on the mask do not have to be as small as the feature sizes desired on the target, are described. The apparatus includes a source of coherent radiation having a chosen wavelength directed onto a focusing optic, the reflected converging light passing through a Talbot mask and impinging on a target substrate.

RELATED CASES

The present patent application claims the benefit of Provisional PatentApplication Ser. No. 61/638,760 filed on 26 Apr. 2012 for “Non-Contact,Scalable and Defect Free Optical Nano-patterning by Demagnified TalbotEffect” by Mario C. Marconi et al., the disclosure and teachings ofwhich are hereby incorporated by reference herein.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support from the NationalScience Foundation under Grant Numbers ECCS-0901806 and EEC-0310717. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to printing arraysof nano-scale structures using an illuminated Talbot mask and, moreparticularly, to illuminating a Talbot mask with convergent coherentlight such that the Talbot image is demagnified, thereby enablingprinting of arrays of nanostructures having dimensions smaller thanthose present in the mask.

BACKGROUND OF THE INVENTION

The ability to print nano-scale structures in a cost effective andconvenient way will enable more rapid advances in nanotechnology. Asexamples, fabrication of templates for memory chips, surface enhancedRaman scattering detectors, and arrays of nano-antennas would benefitfrom such advances.

SUMMARY OF THE INVENTION

Embodiments of the present invention overcome the disadvantages andlimitations of prior art by providing an apparatus and method forprinting arrays of unit cells or tiles having nanometer dimensions inthe surface of a photoresist.

Another object of embodiments of the invention is to provide anapparatus and method for printing arrays of unit cells or tiles havingnanometer dimensions in the surface of a photoresist by illuminating aTalbot mask with a coherent beam of extreme ultraviolet light.

Yet another object of embodiments of the invention is to provide anapparatus and method for printing arrays of unit cells or tiles havingnanometer dimensions in the surface of a photoresist by illuminating aTalbot mask with a coherent beam of extreme ultraviolet light, such thatthe feature sizes on the mask do not have to be as small as the featuresizes desired on the target.

Still another object of embodiments of the invention is to provide anapparatus and method for printing arrays of unit cells or tiles havingnanometer dimensions in the surface of a photoresist by illuminating aTalbot mask with a coherent beam of extreme ultraviolet light, such thatthe feature sizes on the mask do not have to be as small as the featuresizes desired on the target substrate, and unit cell averagingeliminates or at least substantially reduces the number of mask defectstransferred onto the target.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the apparatus for producing nanometer-scale patterning on thesurface of a sample hereof includes: a light source for generating acoherent light beam having a chosen wavelength; a focusing optic intowhich the light beam from the light source is directed, for causing thelight beam emerging therefrom to converge; and a Talbot mask having aperiodic pattern of nanometer-scale unit cells formed thereon, theTalbot mask disposed in the converging light beam from the focusingoptic at a first chosen distance from the focusing optic such that theconverging light beam passes therethrough; the sample being disposed ata second chosen distance from the Talbot mask, and having aphotosensitive surface thereon responsive to the light beam passingthrough the Talbot mask, for generating a nanostructure having thepattern of unit cells of the Talbot mask on the sample.

In another aspect of the present invention, and in accordance with itsobjects and purposes, the method for producing nanometer-scalepatterning on the surface of a sample hereof includes: directing acoherent light beam having a chosen wavelength onto a focusing opticsuch that the light beam emerging therefrom converges; placing a Talbotmask having a periodic pattern of nanometer-scale unit cells formedthereon in the converging light beam at a first chosen distance from thefocusing optic, wherein the converging light beam passes through theTalbot mask; and placing the sample at a second chosen distance from theTalbot mask, the having a photosensitive surface thereon responsive tothe light beam passing through the Talbot mask, for generating ananostructure having the pattern of unit cells of the Talbot mask on thesample.

Benefits and advantages of the present invention include, but are notlimited to, an apparatus and method for nanopatterning of substratesusing the demagnified Talbot effect such that: (a) large arrays ofnanostructures can rapidly be printed; (b) short extreme ultravioletwavelengths permits sub-100 nm spatial resolution; (c) thede-magnification factor can be continuously adjusted, that is,continuously scaled; (d) the patterning is the effect of the collectivediffraction of numerous tiled units that constitute the periodic array,giving rise to error resistance such that a defect in one unit isaveraged over the area of the mask wherein the print does not show anydefects; (e) the Talbot mask does not wear out since the method isnon-contact; and (f) the feature sizes on the mask do not have to be assmall as the feature sizes desired on the target substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate embodiments of the apparatus of thepresent invention and, together with the description, serve to explainthe principles of the invention. In the drawings:

FIG. 1 is a schematic representation of a perspective view of anembodiment of apparatus of the present invention, showing a coherentlight beam having a chosen wavelength being directed onto a focusingoptic, illustrated as a spherical mirror in FIG. 1, the reflectedconverging light passing through a Talbot mask and impinging on a targetsample.

FIG. 2 is a graph of the calculated demagnification factor p′/p as afunction of the distance from the focusing mirror to the Talbot maskdivided by the focal length of the focusing mirror, the circles showingexperimental results using the apparatus illustrated in FIG. 1, hereof.

FIG. 3 shows atomic force microscope scans of printed features obtainedusing different demagnifications (p′/p); 0.98 (scan a), 0.887 (scan b)and 0.865 (scan c), corresponding to the three data points (circles)plotted in FIG. 2 hereof.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the nano-scale printing technology of the presentinvention demonstrate that arrays of units cells (or tiles) withnanometer dimensions can be printed in the surface of a photoresist byilluminating a mask with a coherent beam of extreme ultraviolet laserlight. A generalization of the classical self-imaging Talbot effect,where the periodic structure was a series of lines, to the printing ofcomplex images (tiles having arbitrary designs) arranged in a squarematrix is described in “Nanometer scale Talbot patterning with a tabletop soft X-ray (EUV) laser,” by A. Isoyan et al., Journal of VacuumScience and Technology B, B27, 2931-2936, (2009), by which a periodicstructure illuminated by a coherent light beam is reproduced at certainplanes located at distances equal to multiples of a defined distanceknown as the “Talbot distance”, produces self-images at these locationswhich are the product of the collective contribution of the diffractionof the individual cells (or tiles) in the mask. The distance between thesample and the mask is determined by the periodicity of the mask and thewavelength according the relationship:

${z_{nT} = \frac{n\; 2\; p^{2}}{\lambda}},$where n is the dimensionless Talbot plane order, p is the periodicity(distance) of the tiling in the mask and λ is the wavelength ofillumination. The invention enables printing of arrays of nanostructureshaving dimensions approximately 85% the size of the structures presentin the mask (about 15% smaller). In accordance with embodiments of thepresent invention, demagnification of the Talbot image is achieved byilluminating the Talbot mask with a convergent coherent light beam inplace of a collimated light beam.

Reference will now be made in detail to the present embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. In the FIGURES, similar structure will be identified usingidentical reference characters. It will be understood that the FIGURESare for the purpose of describing particular embodiments of theinvention and are not intended to limit the invention thereto. Turningnow to FIG. 1, a schematic representation of a perspective view of anembodiment of apparatus, 10, of the present invention, showing laserbeam, 12, having a chosen wavelength and generated by light source, 14,being directed onto focusing optic, 18, illustrated in FIG. 1 as aspherical mirror, but could also be a lens or other optical device aswould be known to one having skill in the art at the time the inventionwas made. The wavelength of the laser may be varied, typically fromabout 8 nm to approximately 47 nm, the range were coherent light sourceshaving sufficient energy for developing photoresists are presentlyavailable. Shorter wavelengths are generally better suited forreproduction of smaller mask features, with wavelengths of ≦50 nm beinguseful for nanometer-scale mask features (generally between about 1 nmand about 100 nm). Although responses of various photoresist materialsare only weakly dependent on wavelength, for wavelengths below about 50nm, most photoresists are sensitive. Typical photoresist materials thatmay be used include polymethylmethacrylate (PMMA) and hydrogensilsesquioxane (HSQ); however, there are many commercially availableproprietary compositions.

The relevant parameters for the illuminating light beam are wavelength,which determines the resolution of the printed features, and spatialcoherence, which determines the area that can be printed. Spatialcoherence is characterized or defined in terms of the “coherence area”or “coherence radius”. A “coherence area” larger or equal to the area ofthe Talbot mask (or an equivalent “coherence radius” equal or largerthan W/2, with W the size of the mask) is required for Talbot imaging.

The focal distance of the focusing optic may be varied, with typicalvalues being between about 25 cm and 1 m. When a spherical mirror isemployed, the mirror simultaneously serves as wavelength filter andproduces a convergent illumination beam. That is, the reflectivity ofthe mirror is adjusted to peak in the vicinity of the incident laserwavelength, and partially rejects background continuum from the plasmasthat are the active media of the extreme ultraviolet (EUV) lasersemployed. Mirror coatings are designed specifically for each wavelength;for example, at 47 nm Si/Sc is used while at 13 nm, Si/Mo providesbeneficial results.

In other embodiments (such as use of a focusing lens in place of aspherical mirror), the addition of a wavelength filter may bebeneficial, but is not necessary, and will reduce background noise atother wavelengths. As one having skill in the art will understand, theneed to improve the signal-to-noise ratio of the laser beam will dependon several factors, including the background noise intrinsic to thelaser used and the sensitivity of the photoresist to the backgroundwavelengths for photoresist processing.

Converging laser beam 16 is directed by focusing optic 18 onto Talbotmask, 20, and, after passing through mask 20, onto sample target, 22,where arrays disposed on mask 20 are to be printed. A Talbot mask is asemi-transparent structure constructed of materials chosen depending onthe intended wavelength of use, having a periodic tiled pattern formedtherein, using standard electron beam lithography, as an example. Theperiodic tiled structure is chosen to be as opaque as possible to theincident light, while the supporting membrane or other structure ischosen to be as transparent as possible thereto. An example for 47 nmlight is a cured, opaque photoresist material disposed on a siliconnitrate membrane between about 25 nm and approximately 30 nm. Forshorter wavelengths (approximately 10 nm), an approximately 100 nm thicksilicon nitrate membrane (transparent) with about 50 nm chromium(opaque) for the tiles, may be used.

Typically, target 22 is coated with a photosensitive material such as aphotoresist, with PMMA or HSQ being examples, and is located behindTalbot mask 20 is generally separated from mask 20 by a distance, 24,corresponding to a Talbot plane. There are an infinite number of Talbotplanes which could be selected and the operation of embodiments of theinvention encompasses any choice of Talbot plane. However, as thedistance between Talbot mask 20 and target 22 increases, the numericalaperture and resolution both decrease. Although the demagnificationfactor increases slightly with higher order Talbot planes, this effectis generally offset by the decrease in resolution. For these reasons,typically the target will be placed at the first Talbot plane. Thelocation of the Talbot planes is determined by the above formula forz_(nT).

The present invention is not limited to the basic Talbot effect but canalso exploit what is known as the “fractional” Talbot effect orMontgomery effect (See, e.g., W. Montgomery, “Self-Imaging Objects ofInfinite Aperture,” J. Opt. Soc. Am. 57, 772-775(1967)) by placing thetarget at specific fractions of Talbot intervals. The Montgomery effectpermits objects having two different periodic structures embedded in asingle array to replicate themselves at distances that are fractions ofthe traditional Talbot distance. By exploiting the Montgomery effect, itis possible to adjust the periodicity of the images produced by theTalbot mask. At the Talbot distance, the image has the same periodicityas that of the features of the mask. For fractional Talbot distances,the image has a periodicity that is a multiple of the originalperiodicity of the mask. However, the contrast is diminished, and forprinting purposes it may be a limitation (insufficient intensitydifference between black and bright regions). The use of this effectdepends on the design of the tiles. As the Montgomery effect is a wellunderstood optical phenomenon, those having skill in the art willunderstand the relationships between possible fractions of Talbotintervals and the resulting effect on the periodicity. For example,double period images are obtained between the Talbot distances at bothone quarter and three quarters of a Talbot length (also referred to asTalbot interval). Aligning the sample at either of these intervalstherefore allows the possibility to double the period.

The mask-sample combination 20-22 may be placed along the path ofconverging beam 16. The demagnification factor can be continuouslyvaried by maintaining a constant mask-sample distance and movingmask-sample combination along converging laser beam 16 between focusingoptic 18 and its focal plane, 26. The closer the set mask-sample islocated to focal plane 26, the smaller the print size (higherdemagnification). The dependence of the demagnification factor on thedistance between mask 20 and focal plane 26 is not linear. Thedemagnification factor increases at a larger rate when the mask is movedcloser to the focal plane. Additionally, the demagnification factor hasa different dependence for the different Talbot planes. For higher-orderTalbot planes, the significant dependence of the demagnification factoron the distance from the focus is observed over a larger range ofdistances. The distance dependence of the demagnification factor is alsoinfluenced by the focal length of focusing optic 18.

In one embodiment, extreme ultraviolet (EUV) laser beams are employed.Because the wavelength of EUV radiation has nano-scale dimensions,embodiments of the invention can be used to print arrays of structuresof nano-scale dimensions. The EUV tabletop laser described in the paperby A. Isoyan et al., supra, is highly spatially coherent, and istherefore useful in practicing the invention. The apparatus permits acontinuously scalable replication of a periodic mask in the surface of aphotoresist, where the resolution of the image on the target is definedby:

${\Delta_{n} = {\frac{\lambda}{2}\sqrt{1 + \left( \frac{2\;{np}^{2}}{\lambda\; W} \right)^{2}}}},$where W is the size of the Talbot mask (in units of length), n is theTalbot plane order, p is the periodicity (in units of length) of thetiling in the mask and λ is the wavelength of illumination.The demagnification factor can be calculated with the followingexpression:

${\frac{p^{\prime}}{p} = \frac{z}{f - s}},$where p′ is the periodicity of the tiling in the target, z is thedistance between the target and the focal plane, f is the focal distanceof the focusing element (lens or mirror) and s is the distance betweenthe focusing element and the mask (FIG. 1). The demagnification factordepends strongly on the distance s as it approaches the focal length f.A plot of the calculated de-magnification factor p′/p with the threeexperimental results indicated as circles is shown in FIG. 2. With thedistance between the mask and the target being held constant, as shownin FIG. 1, as s/f approaches unity, p′/p, falls steeply. The slope ofthis decline becomes less steep when the target is placed at higherorder Talbot planes (although using higher order Talbot planes decreasesthe resolution).

The possibility of printing a reduced (demagnified) image relaxes therequirements in the fabrication of the mask. Not only doesdemagnification mean that the feature sizes on the mask do not have tobe as small as the feature sizes desired on the target, but the unitcell averaging eliminates or at least substantially reduces the numberof mask defects transferred onto the target. The resolution that can beachieved with this technique is dictated by the numerical aperture ofthe exposure apparatus. Laser noise is unimportant since the intensityof the laser is much greater than the background. Further, by adjustingthe laser dose, one can print only that portion of the light thanderives from the laser, whereby the background light is insufficient toactivate the photoresist.

Photoresist exposure time, laser power and other parameters necessaryfor effective photolithographic printing will be determined by factorssuch as the photoresist used and the requirements of the user. As anexample, using a 46.9 nm capillary discharge laser for the illuminationand HSQ as the photoresist, it is necessary to apply at least 14 mJ/cm²in order to begin the photolithographic activity (See, e.g., “Nanoscalepatterning in high resolution HSQ photoresist by interferometriclithography with table top EUV lasers,” by P. W. Wachulak et al.,Journal of Vacuum Science and Technology B25 (6), 2094, (2007)). Thisdose is approximately a factor of 5 smaller than that required for usingPMMA as the photoresist.

Although the invention disclosed herein contemplates applicationsincluding photolithography, the invention may be generalized toaccommodate any specific photolithographic procedures that those skilledin the art will already be familiar with. Another advantage of thisnanopatterning technique is that because these unit cells are replicatedmany times in the plane of the mask, any defect in any of the unitarycells is averaged over the very large numbers of tiles in the mask. Thearea of the defect is thus negligible as compared with the area of themask consequently generating a virtually defect-free image on the targetplaced on a Talbot plane.

Having generally described the invention, the following EXAMPLE providesgreater detail.

EXAMPLE

A tabletop extreme ultraviolet (EUV) capillary discharge laser emittingat 46.9 nm, the operation of which is described in C. D. Macchietto etal., “Generation of millijoule-level soft-x-ray laser pulses at a 4-Hzrepetition rate in a highly saturated tabletop capillary dischargeamplifier,” Optics Letters. 24, (1999), pages 1115-1117, was utilized asthe light source in the apparatus of FIG. 1, hereof. The EUV laser beamwas reflected by a spherical mirror with a focal length of 25 cm and areflectivity of approximately 40% (using a multilayered silicon/scandiumcoating) that simultaneously serves as wavelength filter and produces aconvergent illumination beam. The Talbot mask and a Si wafer coated withPMMA were disposed in the converging light beam. The Si wafer wasseparated from the Talbot mask by a distance corresponding to the firstTalbot plane, and the mask-sample combination was located along the pathof the converging beam at chosen distances from the focusing optic. Thesample was exposed with 200 laser shots, each one having an averageenergy of 300 μJ. Focusing the light beam provides the advantage ofincreasing the energy density deposited in the sample; however, focusingcompromises the spatial coherence of the illumination which has adetrimental effect on the resolution of the printing. FIG. 3 showsatomic force microscopy scans of different prints obtained with thetarget at 22 cm, 24.4 cm and 24.5 cm from the mirror showing thede-magnification factors p′/p of: 0.98 (scan a), 0.887 (scan b) and0.865 (scan c), respectively, corresponding to the three data points(circles) plotted in FIG. 2 hereof.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. The embodiments were chosen and described in order tobest explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the invention be defined by the claims appended hereto.

What is claimed is:
 1. A method for producing nanometer-scale patterningon the surface of a sample, comprising: directing a coherent light beamhaving a chosen wavelength onto a focusing optic such that the lightbeam emerging therefrom converges placing a Talbot mask having aperiodic pattern of nanometer-scale unit cells formed thereon in theconverging light beam at a first chosen distance from the focusingoptic, wherein the converging light beam passes through the Talbot mask;and placing a sample disposed at a second chosen distance from theTalbot mask, having a photosensitive surface thereon responsive to thelight beam passing through the Talbot mask, for generating ananostructure having the pattern of unit cells of the Talbot mask on thesample.
 2. The method of claim 1, wherein the second chosen distance isthe distance of the first Talbot plane.
 3. The method of claim 2,wherein the second chosen distance is one-fourth or three-fourths of thedistance of the first Talbot plane.
 4. The method of claim 1, whereinthe first chosen distance is the distance at which a chosendemagnification of the pattern of arrays of unit cells occurs.
 5. Themethod of claim 1, wherein the focusing optic comprises a mirror.
 6. Themethod of claim 1, wherein the light source comprises an extremeultraviolet laser.
 7. The method of claim 1, wherein the photosensitivesurface comprises a photoresist.